You can use both Water Shuriken and Giant Water Shuriken in the same turn since Greninja BREAK has both Abilities. Each requires a separate discard.
Notably, Pokemon R&D just ruled that you cannot use Water Shuriken, evolve into Greninja BREAK, and then use Water Shuriken again in the same turn. Although Greninja BREAK is a new Pokemon, when it "retains" the Ability it keeps track of the fact that it has already been used this turn. You'd be free to use Giant Water Shuriken once you evolve, of course.